The control of the α-Al2O3 polymorph in industrial scale was achieved in the beginning of the 1990's with commercial products based on U.S. Pat. Nos. 5,137,774 . 5,654,035, 5,980,988, 7,442,431 and 5,863,640 describe the deposition of α-Al2O3 coatings with preferred fiber textures. U.S. Pat. No. 6,333,103 describes a modified method to control the nucleation and growth of α-Al2O3 on the (1 0 10) plane. U.S. Pat. No. 6,869,668 describes a method to obtain a strong (1 0 0) texture in α-Al2O3 using ZrCl4 as a texture modifying agent. The prior-art processes discussed above use deposition temperatures of about 1000° C. U.S. Pat. No. 7,094,447 describes a technique to achieve a pronounced (0 1 2) texture. Deposition techniques to produce pronounced (1 0 4) and (1 1 6) textures are disclosed in U.S. Pat. Nos. 7,442,432 and 7,455,900, respectively. U.S. Pat. Nos. 7,993,742 and 7,923,101 disclose (0 0 1) textured alumina layers. The (0 1 2), (1 0 4) and (0 0 1) textured alumina layers are compared and the (0 0 1) texture is found superior over the other textures. In the above-mentioned disclosures the textures of the α-Al2O3 layers were determined by using XRD and quantified by calculating the texture coefficients (TC) using the Harris formula. In U.S. Pat. Nos. 7,763,346 and 7,201,956 EBSD has been used to define the (0 0 1) texture, and the texture is quantified by the inclination of the basal (0 0 1) plane with respect to the coating surface.
Textured α-Al2O3 layers are composed of columnar grains and the α-Al2O3 grains are terminated by facets having a relatively large angle to the substrate surface resulting in a rough surface morphology.
U.S. Pat. No. 7,923,101 describes some degree of flattening of the surface of an α-Al2O3 layer with a (0 0 1) texture. US 2012/003452 describes that flat-top surfaces have been obtained in α-Al2O3 layers containing one or more elements selected from a group consisting of Ti, Y, Zr, Cr and B. These layers are deposited on an α-Al2O3 layer, which in turn is deposited on a heat-treated thin Al2O3 film.
U.S. Pat. No. 7,597,511 discloses a surface-coated cutting tool comprising a substrate of cemented carbide or titanium-carbonitride-based cermet with a hard coating layer including a lower layer of a Ti compound with a total average layer thickness of 3 to 20 μm, and an upper AlZrO layer being formed in a 2-step CVD process requiring ZrCl4, the upper layer being described as “reformed” AlZrO layer. The “reformed” AlZrO layer was shown to have a structure with crystal grains having an even polygonal shape with a large particle size in the plane perpendicular to the thickness direction and an elongate shape in the thickness direction with an even surface. The coating is described to exhibit good cutting performance, however, the deposition of Zr oxide in addition to Al oxide is generally known to increase production costs and putting high demands on operational safety due to self-flammability of freshly polished Zr.